Modern backplanes, also referred to as motherboards, serve as communication media for the exchange of electronic signals between a plurality of daughter cards. The daughter cards generate communication signals, for example, data signals, address signals, and control signals which are distributed to daughter card connectors mounted on one or both sides of each daughter card. The daughter card connectors register with a corresponding set of backplane connectors on the backplane, which in turn distributes the signals between daughter cards along various communication paths.
Each connector pair includes an array of conductive interconnects in the form of mating male pins and female contacts which couple by frictional contact. The interconnects each provide a separate electrical path for the transmission of signals between, boards typically with some providing the transmission of signals in one direction and the others providing the transmission of signals in the other direction. Within a connector, the interconnect paths run substantially parallel.
As communication technology improves, there is increasing demand on connectors to channel more data through a given area. An obvious solution is to reduce the distance between signal paths, allowing for more data channels. This, however, increases the likelihood of electromagnetic coupling between signals. Such coupling generally takes on two forms, namely electric field E (capacitive) coupling and magnetic field H (inductive) coupling. The influence of either form of coupling between signals is generally referred to in the art as crosstalk. Crosstalk corrupts the waveform of an affected signal, which, in turn, can cause data errors, timing errors or other anomalies which interfere with proper data communication.
The danger of crosstalk is most significant where signals converge in a densely-populated region as in a connector. This passage of signals through connector pins in close proximity to each other makes crosstalk inevitable in prior art connector configurations, for example the configuration shown in the perspective view of Prior Art FIG. 1A and the side view of Prior Art FIG. 1B. This configuration includes a male connector housing 30 having an array of male pins 36 mounted to a, backplane 32 and a corresponding female connector housing 34 having a corresponding array of female contacts 38 bonded to a printed circuit board daughter card 42. The female contacts 38 connect to the printed circuit board 42 by metal rods 40 which are bent approximately at right angles 44 contacting the printed wiring through plated through-holes 46 or surface mount pads.
Such an arrangement is sufficient for transporting signals of moderate speeds. However, in modern systems having faster signal clocks and increased data throughput, crosstalk interferes with system performance. In particular, the length of the paths between the backplane 32 and the daughter card 42 and the coupling between them introduces delays, distortions and unwanted couplings which seriously degrade the information transmitted.
Prior Art FIG. 1B illustrates the path of a signal, represented by arrow 48, propagating between a backplane 32 and a daughter card 42 through a prior art connector assembly 30, 34. It is apparent that the path length of the conductive medium between boards, including male pin 36A, female contact 38A and metal rod 40A, is extended and linear. It is also apparent that this path is parallel to adjacent paths defined by male pin 46B, female contact 38B and metal rod 40B over its entire length. The behavior of the signal current 48 and its responsibility in inducing crosstalk are illustrated in Prior Art FIG. 2 and FIG. 3.
Prior Art FIG. 2 illustrates signal current 48 propagating through a male pin 36, entering a female contact 38 at contact point 52 and passing to conducting rod 40. As the signal propagates, it generates an H field 53 represented in the drawing in exemplary fashion as entering the plane of the page at points 49 and exiting the plane of the page at points 51. The H field 53 is illustrated in the perspective view of Prior Art FIG. 3. E fields are not shown, but they are also generated by the voltages on the conductors.
In Prior Art FIG. 3, H fields 53A, 53B, 53C respectively generated by signal currents 48A, 48B, 48C emanate in a generally cylindrical orientation about the signal path, with the circles representing each field at a particular axial location along the conductive paths as shown. Depending on the magnitude and frequency of the current and the relative proximity of the signal paths, the resultant H field 53A generated by one signal 48A may extend spatially far enough to influence a signal 40B of an adjacent path and a signal 40C of a non-adjacent path. This form of coupling is referred to in the art as inductive coupling. Furthermore, the electric field created by the first signal 48A, for example, may couple to nearby signal paths 48B, 48C. This is known in the art as capacitive coupling. In this manner each of signals 48A, 48B, 48C may influence adjacent or non-adjacent signals.
It is well known in the art that a conductive medium has an inherent inductance caused by an H field generated about the medium by the current flowing through it. The closer a first medium is placed in proximity to a second medium, the more likely their respective H fields will influence each other. This, in turn, leads to an increased likelihood of crosstalk between media.
The theory of crosstalk in transmission lines is somewhat involved, but for printed circuit board connectors, the transmission line paths between a backplane and daughter card through male pins and female contacts are sufficiently short such that the signal propagation time is currently generally less than one-half of the rise time of the digital signals transmitted thereon. For this condition, the crosstalk amplitude increases as the signal rise time or frequency component increases. For the same reason, crosstalk increases with connector path length. Furthermore, the male pin/contact paths are characteristically inductive, causing increased signal attenuation as frequency increases. To accommodate high frequencies, or fast rise times, it is common to insert coaxial contact pairs in backplane to daughter card connectors. However, because coaxial pairs are expensive and bulky, they are used only in extraordinary circumstances.
The controlled-impedance lines of predominantly inductive prior art connectors are generally not matched between the backplane and daughter card, causing reflections when signal rise times approach the propagation delay of the connector paths. This causes signal distortion and attenuation and increases crosstalk due to multiple reflections, limiting high-frequency throughput. Shielded connectors are available to enhance throughput, but generally are expensive to produce and have relatively poor contact density per unit area. To achieve reduction of H and E fields at high frequencies, a shield is typically placed between each row of contacts on eat side (male and female) of the connector, a very complicated and expensive configuration. In this configuration, H field attenuation is provided by providing ground return paths adjacent each forward signal path E field attenuation is not fully effective because the resultant shield geometry is suboptimal. This phenomenon is detailed in Hybricon's Technology Focus publication H89107, incorporated herein by reference, which explains crosstalk for two parallel conductive paths.
Prior Art FIG. 15A is a side view of a conventional shielded connector, illustrating current flow in adjacent signal paths. For purposes of example, a backplane 32 includes a male connector 30 and a daughter card includes a corresponding female connector 34. Planar shields 202 are inserted between rows or columns (for example along the vertical cross section of FIG. 1B) of mated contacts 36A, 40A, and 36B, 40B, in such a manner that return currents I.sub.R between mating boards will pass through the shields, and thereby cancel or attenuate the H fields generated by the forward currents I.sub.F through the signal contacts. E fields generated by the voltage on the contacts are likewise terminated by the shields 202. This is analogous to the manner by which the E fields and H fields are contained by stripline traces on a printed circuit board.
In another technique for enhancing H field attenuation, signals are configured in the connector such that the nearest paths to a given forward signal path are return paths. This causes the H fields of nearest paths to be in opposite orientations, thereby tending to reduce the overall H fields which couple to other paths. The opposite potential also tends to reduce the respective E fields. This technique is somewhat effective but results in the wasteful use of pins as the return paths that are generally connected to the ground plane.
These techniques are somewhat effective due to the partial cancellation of the H and E fields between the forward and return paths of each of the signals. However, both approaches reduce signal contact density, which reduces the usefulness of the connector, and raises the cost per signal line through the connector. Other connectors from Vendors such as AMP, Augat, and Teradyne employ complicated stripline contacts to improve performance. However, such connectors are relatively expensive to produce.